Methods of and apparatus for making coated optical fiber

ABSTRACT

In the manufacture of coated optical fiber, fiber (21) is drawn from a preform (22) and coated with one or preferably two layers (42,44) of light curable coating materials. Afterwards, the coating materials are cured. Increases in manufacturing line speed may be achieved if the cure speed of the coating materials is increased. This is accomplished by the simultaneous application of a magnetic field during irradiation of the curable coating materials to enhance the crosslinking of the coating materials by a free radical polymerization mechanism. Upon absorption of light, a photoinitiator in each composition cleaves to produce two free radical fragments in the spin paired or singlet state. The magnetic field has the effect of enhancing the production of spin parallel radicals which enhances the polymerization initiation of the coating material, thereby allowing an increase in the manufacturing line speed through drawing and coating apparatus.

TECHNICAL FIELD

This invention relates to methods of and apparatus for making coatedoptical fiber. More particularly, this invention relates to methods ofand apparatus for coating optical fiber with enhanced cure speed.

BACKGROUND OF THE INVENTION

In the manufacture of optical fiber, a glass preform rod which generallyis manufactured in a separate process is suspended vertically and movedinto a furnace at a controlled rate. The preform softens in the furnaceand optical fiber is drawn freely from the molten end of the preform rodby a capstan located at the base of a draw tower.

Manufacturers of optical fiber each have adapted a particular techniquefor the manufacture of preforms from which the optical fiber is drawn.These include outside vapor deposition and modified chemical vapordeposition, for example.

Because the surface of the optical fiber is very susceptible to damagecaused by abrasion, it becomes necessary to coat the optical fiber,after it is drawn, before it comes into contact with any surface.Inasmuch as the application of a coating material must not damage theglass surface, the coating material is applied in a liquid state. Onceapplied, the coating material must become solidified rapidly before theoptical fiber reaches the capstan. This may be accomplished byphotocuring, for example.

Those optical fiber performance properties which are affected most bythe coating material are strength and transmission loss. Coating defectswhich may expose the optical fiber to subsequent damage arise primarilyfrom improper application of the coating material. Defects such as largebubbles or voids, non-concentric coatings with unacceptably thinregions, or intermittent coatings must be prevented. The problem ofbubbles in the coating material has been overcome. See, for example,U.S. Pat. No. 4,851,165 which issued on July 25, 1989 in the names of J.A. Rennell, Jr. and C. R. Taylor. Intermittent coating is overcome byinsuring that the fiber is suitably cool at its point of entry into thecoating applicator to avoid coating flow instabilities. Coatingconcentricity can be monitored and adjustments made to maintain anacceptable value.

Optical fibers are susceptible to a transmission loss mechanism known asmicrobending. Because the fibers are thin and flexible, they are readilybent when subjected to mechanical stresses, such as those encounteredduring placement in a cable or when the cabled fiber is exposed tovarying temperature environments or mechanical handling. If the stressesplaced on the fiber result in a random bending distortion of the fiberaxis with periodic components in the millimeter range, light rays, ormodes, propagating in the fiber may escape from the core. These losses,termed microbending losses, may be very large, often many times theintrinsic loss of the fiber itself. The optical fiber must be isolatedfrom stresses which cause microbending. The properties of the opticalfiber coating material play a major role in providing this isolation,with coating geometry, modulus and thermal expansion coefficient beingthe most important factors.

Typically two layers of coating materials are applied to the drawnoptical fiber. Furthermore, two different kinds of coating materials areused commonly. An inner layer which is referred to as a primary coatingmaterial is applied to be contiguous to the optical glass fiber. Anouter layer which is referred to as a secondary coating material isapplied to cover the primary coating material. Usually, the secondarycoating material has a relatively high modulus, e.g. 10⁹ Pa, whereas theprimary coating material as a relatively low modulus such as, forexample, 10⁶ Pa. In one arrangement, the primary and the secondarycoating materials are applied simultaneously. Such an arrangement isdisclosed in U.S. Pat. No. 4,474,830 which issued on Oct. 2, 1984 in thename of C. R. Taylor.

Subsequently, both the inner and the outer layers of coating materialsare cured beginning from the outside progressing inwardly. Alsotypically, the primary and the secondary coating materials compriseultraviolet light curable materials each being characterized by aphotoactive region. A photoactive region is that region of the lightspectrum which upon the absorption of curing light causes the coatingmaterial to change from a liquid material to a solid material. Both thematerials which have been used for the primary and for the secondarymaterials have comparable photoactive regions. Because the photoactiveregions are comparable, the curing light for the primary coatingmaterial will be attenuated by the secondary coating material. As aresult of the attenuation, less light reaches the primary coatingmaterial.

Of course, notwithstanding the attenuation of the curing light by thesecondary coating material, it is important that the primary coatingmaterial be fully cured. This problem has been overcome in the prior artby reducing the line speed to allow longer exposure time of the primarycoating material to the ultraviolet curing light energy inasmuch as theultraviolet curing light energy is inversely proportional to line speed.

Although the foregoing solution is a workable one, it has itsshortcomings. Most importantly, any reduction in line speed is notdesirable and runs counter to current efforts to increase draw lengthsand to increase substantially draw speeds of the optical fiber.

What is needed and seemingly what is not disclosed in the prior art is acoated optical fiber which overcomes the foregoing problem ofattenuation by the secondary coating material of the light energy usedto cure the primary coating material without compromising line speed.

The foregoing problem has been exacerbated because, presently, opticalfiber manufacturers are attempting to provide larger preforms to allowthe drawing of a longer length of fiber from each preform. Also, thereis a strong desire to decrease the time required to cure the coatingmaterials. Should this desire be realized, then increases in line speedcould be achieved.

Any solution to the problem of increased cure speed desirably isaccomplished without changing the composition of the coating materials.Should those materials be changed, expensive, time consuming testingwould have to be carried out to requalify the coating system. Also, anyincrease in cure speed desirably should be carried out without theaddition of curing lamps or without the lengthening of the curingportion of the manufacturing line.

More particularly, what is needed and what seemingly is not available inthe prior art is a coating arrangement for increasing the cure speed ofcoating materials for optical fiber. The sought-after methods andapparatus should be capable of being integrated into existing opticalfiber draw lines and should be able to be implemented without the needto change the chemical composition of the coating materials of thecoating system.

SUMMARY OF THE INVENTION

The foregoing problems of the prior art have been overcome by themethods and apparatus of this invention. In a method of making opticalfiber, fiber is drawn from a preform and provided with one or preferablytwo layers of coating material. Each of the coating materials is anultraviolet light curable composition of matter or a visible lightcurable composition may be used. After the coating materials have beenapplied to the drawn fiber, the coating materials are cured. Then thedrawn coated fiber is taken up.

In order to increase the cure speed of the coating layers, the coatingmaterials are exposed to a magnetic field at the same time they areexposed to ultraviolet light. The magnetic field has the effect ofenhancing the production of spin parallel radicals from a photoinitiatorwhich has been promoted to its excited state by absorption ofultraviolet light. As a result, the polymerization initiation of thecoating material is enhanced, allowing the overall cure speed to beincreased.

BRIEF DESCRIPTION OF THE DRAWING

Other features of the present invention will be more readily understoodfrom the following detailed description of specific embodiments thereofwhen read in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view of a manufacturing line for drawing andcoating optical fiber;

FIG. 2 is an end view in section of an optical fiber having a coatingsystem thereon;

FIG. 3 is a diagram to show a free radical polymerization mechanism fora photoinitiator;

FIG. 4 is a perspective view partially in phantom of a curing chamber inwhich coating materials on drawn optical fiber are cured and exposed toa magnetic field during cure;

FIG. 5 is a graph which depicts equilibrium modulus versus dose of anoptical fiber UV curable ink with and without being subjected to amagnetic field during cure;

FIG. 6 is a graph which depicts equilibrium modulus versus dose of anoptical fiber ultraviolet light curable coating material with andwithout being subjected to a magnetic field during cure;

FIGS. 7 and 8 are graphs depicting equilibrium modulus versus dose oftwo other optical fiber ultraviolet light curable coating materials withand without being subjected to a magnetic field during cure;

FIGS. 9 and 10 are histograms which show the increase is equilibriummodulus of optical fiber coating materials due to the application of amagnetic field during cure at increasing levels of UV dose; and

FIG. 11 is a histogram which shows the increase in equilibrium modulusof an optical fiber ink due to the application of a magnetic fieldduring cure at increasing levels of UV dose.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is shown an apparatus which is designatedgenerally by the numeral 20 and in which is used to draw optical fiber21 from a specially prepared cylindrical preform 22 and for then coatingthe drawn fiber. The optical fiber 21 is formed by locally andsymmetrically heating the preform 22, typically 7 to 25 mm in diameterand 60 cm in length, to a temperature of about 2000°C. As the preform isfed into and through a furnace 23, fiber 21 is drawn from the moltenmaterial.

As can be seen in FIG. 1, the elements of the draw system include thefurnace 23 wherein the preform is drawn down to the fiber size afterwhich the fiber 21 is pulled from a heat zone therein. The diameter ofthe fiber 21 is measured by a device 24 at a point shortly after thefiber is formed and this measured value becomes an input into a controlsystem. Within the control system, the measured diameter is compared tothe desired value and an output signal is generated to adjust the drawspeed such that the fiber diameter approaches the desired value.

After the diameter of the optical fiber 21 is measured, a protectivecoating system 25 (see also FIG. 2) is applied to the fiber by anapparatus 27. Preservation of fiber strength requires the application ofthe protective coating, which shields newly drawn fiber from thedeleterious effects of the atmosphere. This coating system must beapplied in a manner that does not damage the surface of the fiber 21 andsuch that the fiber has a predetermined diameter and is protected fromabrasion during subsequent manufacturing operations, installation andservice. Minimizing attenuation requires the selection of a suitablecoating material and a controlled application of it to the fiber. Such acoating apparatus may be one such as that described in priorlyidentified U.S. Pat. No. 4,474,830. Minimizing diameter variation whichin turn minimizes the losses due to misalignment at connector and splicepoints requires careful design of the draw system and the continuousmonitoring and control of the fiber diameter during the drawing and thecoating steps of the process. Then, the coated fiber 21 is passedthrough a centering gauge 28.

After the coating materials have been applied to the drawn fiber, thecoating materials must be cured. Accordingly, the optical fiber havingthe coating materials thereon is passed through a curing chamber 30 forcuring the coating system and a device 32 for measuring the outerdiameter of the coated fiber. Afterwards, it is moved through a capstan34 and is spooled for testing and storage prior to subsequent cableoperations.

In the apparatus 27, the coating system 25 comprising two coatingmaterials is applied to the optical fiber. The coating system 25includes an inner layer 42 (see FIG. 2) which often is referred to as aprimary coating material and an outer layer 44 which often is referredto as a secondary coating material. The coating material of the innerlayer which has a substantially lower modulus than that of the outerlayer, is such that it prevents microbending of the optical glass fiber.On the other hand, the higher modulus outer layer provides mechanicalprotection for the drawn glass fiber.

Each of the coating materials is curable by being exposed to a portionof the light spectrum. It is commonplace to use ultraviolet lightcurable coating materials which are crosslinked by a free radicalpolymerization mechanism. Generally each of the coating materialsincludes an oligomer, a diluent and a photoinitiator. Also included maybe additives such as, for example, antioxidants, adhesion promoters,ultraviolet (UV) light stabilizers, surfactants and shelf lifestabilizers.

A first step in such a polymerization mechanism is the absorption ofincident ultraviolet irradiation by the photoinitiator constituent ofeach coating composition (see FIG. 3). The photoinitiator upon exposureto suitable light energy is promoted to an excited singlet state whichis a precursor for a caged singlet radical pair. Subsequently, thephotoinitiator of each coating material in its excited singlet statecleaves to produce a caged radical pair, which is usually in the spinpaired or singlet state.

In order to initiate polymerization, the two radical fragments mustdiffuse apart from each other and react with other constituents of thefiber coating materials. The radicals must diffuse out of the cage.

Subsequently, the radicals diffuse from one another and interact withcoating material such as acrylates, for example, that can undergo freeradical polymerization. Some of the photoinitiator in the excitedsinglet state is converted to a triplet state by a process which isreferred to as intersystem crossing. This conversion results in apopulation of the excited triplet state. Upon cleavage from the excitedtriplet state, a triplet or spin parallel radical pair is formed.Radicals diffuse from one another to interact with materials such asacrylates, for example, that are capable of undergoing radicalpolymerization.

For polymerization to occur, radicals must move out of an associatedcage by diffusion which is much faster for a triplet radical pair thanfor a singlet radical pair. Because the free electrons of radicals inthe singlet radical pair are spin paired, they are poised to causebonding. In contrast, free electrons in a triplet radical pair are spinparallel. Because spin parallel radicals repulse one another, theirrecombination is reduced. Also, because spin parallel or triplet radicalpairs repulse one another, the rate of diffusion of the caged radicalsproduced from the excited triplet state on the right of FIG. 3 is higherthan for the singlet stage. As a result, the photoinitiator radicalsfrom the triplet radical pairs are more readily available to initiatepolymerization. The diffusion rate of the triplet radicals from oneanother is enhanced to combine with other components to causecrosslinking.

Unfortunately, the efficiency of the diffusion of the two radicalfragments from each other and hence the rate of polymerization isreduced by the propensity of the free radical pair which are in thesinglet state to combine with each other. As a result, a ratedetermining step in the manufacture of optical fiber is the curing speedof the ultraviolet light curable coating materials.

Long sought after has been a way in which to increase the cure speed.One way in which to do this is to increase the rate of polymerization ofthe coating material which has been applied to the drawn optical fiber.

This problem of achieving an increased cure speed has been overcome bythe application of a magnetic field to the optical fiber coatingsimultaneously with the exposure of the two layers of coating materialsto ultraviolet light energy for curing. The exposure to the magneticfield is effective to cause the excited singlet state of thephotoinitiator to be converted to its excited triplet state. The freeradical pair derived from the singlet state will be spin paired and thefree radical pair derived from the triplet state will be spin parallel.Because diffusion efficiency of the triplet radical pair fragments fromone another is substantially greater than the diffusion efficiency ofthe singlet radical pair fragments from one another, the cure speed ofmaterial systems which can undergo free radical polymerization, such asoptical fiber coatings, for example, and hence the production rate ofsystems which can undergo free radical polymerization such as opticalfiber coatings is enhanced greatly by the application of a magneticfield simultaneously with the irradiation curing of the coating layers.

Although recombination can occur in a triplet radical cage through amechanism referred to as spin flip, recombination in a triplet radicalcage is not nearly as apt to occur as in a singlet radical cage. A moreefficient escape of photoinitiator radicals from one another allows amore efficient initiation of polymerization. The diffusion and hence thepolymerization is much more rapid from the triplet radical pair thanfrom the singlet radical pair. When the photoinitiator is subjected to amagnetic field at the same time it is exposed to curing energy, themagnetic field enhances intersystem crossing from the singlet state to atriplet state. The magnetic field may be such as to provide a higherrate of intersystem crossing and thus serves to increase theconcentration of free radicals.

The polymerization of the coating materials is greatly enhanced by theexposure of the coating materials to a magnetic field simultaneouslywith exposure to light curing energy. As a result, the curing of thecoating materials of the inner and outer layers is accelerated, whichadvantageously allows an increase in speed of the fiber draw line.

The curing chamber 30 provides such enhanced curing of the coatingmaterial or materials. Typically, the curing chamber 30 (see FIG. 4) isprovided with a housing 52. Disposed within the housing 52 is a quartztube 54 having a longitudinal axis 56 parallel to that of the housing.The quartz tube 54 is adapted to have the drawn optical fiber 21 movedtherethrough and has an inner diameter of about 2.5 cm and a thicknessof about 1 mm. Also disposed within the chamber 30 is an elongatedquartz halogen lamp 58 which parallels the path of the optical fiber andwhich emits ultraviolet radiation that is used to cure the coatingmaterial or materials. The lamp 58 and the quartz tube 54 through whichthe fiber is moved are located at the focii of elliptical mirrors 59--59to ensure that substantially all of the periphery of a moving opticalfiber is impinged by light emitted by the lamp 58.

The quartz tube 54 through which the optical fiber is moved istransparent to ultraviolet radiation from the lamp. Consequently, theuse of such a tube does not impair the curing of the coating material onthe moving optical fiber. The ultraviolet curing of the coatingmaterials on the optical fiber is accomplished with energy in thewavelength range of about 200 to 400 nm.

Positioned adjacent to the exterior of the curing chamber 30 is a magnetsystem 60. The magnet system may comprise two bar magnets 62--62 orelectromagnets which are aligned with the quartz tube 54. Because themagnets are positioned outside the chamber 30, they do not interferewith the reflected light and hence do not impair the curing step.

Referring now to FIG. 5, there is shown a comparison of the equilibriummodulus of a UV curable ink achieved at different UV dose levels withand without the simultaneous application of a magnetic field. A curvedesignated 70 represents the plot for the curing of the coatingmaterials without exposure to a magnetic field. A curve designated 72represents the plot for the curing of the coating materials with thesimultaneous exposure to a magnetic field. FIGS. 6, 7, and 8 depictdifferent equilibrium modulus levels of optical fiber UV curable coatingmaterials at varying UV dose levels with and without the simultaneousexposure to a magnetic field. In FIGS. 5, 6 and 7, the broken lines atenhanced levels represent coating material subjected simultaneously tocuring light energy and to a magnetic field.

Histograms in FIGS. 9 and 10 depict the percentage increase inequilibrium modulus of optical fiber coating materials with thesimultaneous application of a magnetic field at varying UV dose levels.The histogram in FIG. 11 depicts the percentage increase in equilibriummodulus of a UV curable ink with the simultaneous application of amagnetic field.

Although this invention has been described in terms of enhancing thecure speed of optical fiber coatings, the invention is not so limited.For example, it would be used to alter the cure speed of light curablematerials which have been applied to any substrate material such asfloor coverings, for example.

It is to be understood that the above-described arrangements are simplyillustrative of the invention. Other arrangements may be devised bythose skilled in the art which will embody the principles of theinvention and fall within the spirit and scope thereof.

I claim:
 1. A method of producing optical fiber, said method include the steps of:providing an optical preform; suspending the optical preform from a suspending means, such that the preform can be drawn; drawing optical fiber from the suspended preform while the preform is subjected to fiber-drawing heat energy; applying at least two layers of a light-curable polymerizable coating material to the drawn fiber where the outer of the two layers attenuates curing light energy directed through it to the inner layer; exposing the drawn coated fiber to light energy to cure the coating material and thereby polymerize it; while simultaneously exposing the drawn coated fiber to a magnetic field to thereby increase the speed of the curing of the coating material; and taking up the drawn coated optical fiber.
 2. The method of claim 1, wherein said magnetic field has a strength of at least about 2000 Gauss and is sufficient to afford significant enhancement of polymerization of said at least one coating material.
 3. The method of claim 1, wherein the coating material is one which is converted from a liquid to a solid by free radical polymerization.
 4. The method of claim 3, wherein the coating material is a visible light curable material.
 5. The method of claim 3, wherein the coating material is an ultraviolet light curable material.
 6. The method of claim 1, wherein an inner layer and an outer layer of coating material are applied to the optical fiber.
 7. The method of claim 6, wherein the inner and the outer layers of coating materials are applied simultaneously.
 8. The method of claim 6, wherein the inner and the outer layers of coating materials are applied successively.
 9. The method of claim 1, wherein the optical fiber is exposed to the curing energy for a time suitable for achieving a predetermined dose level. 